Ultrasonic wellbore dewatering device, system and method

ABSTRACT

An ultrasonic device and system is provided for specific application to unloading non-gaseous production (typically mineralized water which may or may not be associated with produced solids and/or hydrocarbon liquids) from gas producing wells. In one embodiment, the system comprises an ultrasonic particle generator bank, including a transformer as needed (geometry of bank varies depending on down hole configurations) with multiple ultrasonic sources for redundancy/longevity and particle formation rate control. The multiple ultrasonic sources may be powered electrically from the surface, or by other means, with a length management conveyance system. The ultrasonic sources may be buoyed at substantially optimal depth below the surface of the non-gaseous production being particlized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/755,262, filed Jan. 22, 2013, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related to the dewatering of gas producing wellboresincluding methods, devices and systems therefor and more specifically toultrasonic devices, methods and systems therefor.

2. Description of the Related Art

Gas producing wells, such as natural gas and including those wells thathave concurrent production of liquids or non-gaseous products such aswater, oil or condensates, are often incapable of clearing these liquidsfrom the well bore. This is especially true in depleted reservoirs andlow-rate gas producing wells. Liquids can accumulate in the well bore asgas is produced. Accumulated liquid exerts backpressure on the producingformation such that flow of gas is reduced or completely restricted.

Natural gas producing wells can experience sporadic or permanentcessation of production if the gas flow velocity is too low to liftslugs or large drops of non-gaseous production from the wellbore. Thisprocess is known in the field as loading. Typically, the non-gaseousproduction comprises mineralized water which may or may not beassociated with produced solids and/or hydrocarbon liquids.

Typical dewatering methods for reducing liquid accumulation in the wellbore and re-establishing a viable gas production rate usually involveusing the reservoir energy to lift out the liquid or by using externalenergy to lift it out. Reservoir energy is used by concentrating thereservoir pressure using mechanical means (i.e. plunger lift) or bytaking advantage of critical flow properties (i.e. velocity strings).These methods are limited in application by the reservoir pressure andflow rates. External energy is used to activate pumps or to periodicallybail, swab or perform coiled tubing cleanouts (CTC). These methods arenot as limited by reservoir pressure or flow rates but may be tooexpensive (in the case of pumps) or technically inadequate (in the caseof periodic removal methods) to be considered an optimal solution forgas producing wells including shallow gas producing wells.

A further problem with external energy sources such as down-hole pumpsis that many pumping methods are labour intensive, require regularattention and generally use expensive equipment to provide an externalsource of lifting capacity to clear the well bore of the liquids. As aresult, these technologies are cost prohibitive, and are often noteconomically viable for low production wells.

Other dewatering technologies have a narrow operating range, and must besuited to each individual well based on well characteristics such aswater gas ratio (WGR), well pressure, and gas flow rate. Thisinformation is often unavailable, and can be highly variable over time.These technologies generally require regular attention from operationsstaff which can be problematic in areas of limited or restricted leaseaccess. The narrow operating range of these dewatering technologiesmeans that they usually fail when well conditions change in such a waythat they are outside of the operating range.

Failure of these technologies results in down time and lost production,and can also require attention from operations staff in order to resumeproduction.

A need therefore exists for a well dewatering method and system thatovercomes at least one of the above mentioned shortcomings associatedwith existing technologies or at least overcomes at least oneshortcoming inherent to existing and potential well dewatering systemsfurther to those described above.

SUMMARY OF THE INVENTION

This invention uses an ultrasonic particle generator to create ultrafine particles of the non-gaseous production thereby allowing gasproducing wells with low gas flow velocities to remain unloaded. Thesmaller the particle of non-gaseous production, the less velocity isrequired to transport it in the gas stream and out of the well bore.Devices in accordance with the invention may also potentially be used toenhance the utility of velocity string tubulars by allowing the velocitystring to sit with its entrance high above the gas/non-gaseous interfacethereby reducing the risk of flooding the velocity string during aperiod of shut-in.

An ultrasonic device and system is provided for specific application tounloading non-gaseous production (typically mineralized water which mayor may not be associated with produced solids and/or hydrocarbonliquids) from gas producing wells. In one embodiment, the systemcomprises an ultrasonic particle generator bank, including a transformeras needed (geometry of bank varies depending on down holeconfigurations) with multiple ultrasonic sources forredundancy/longevity and particle formation rate control. The multipleultrasonic sources may be powered electrically from the surface, or byother means, with a length management conveyance system. The ultrasonicsources may be buoyed at substantially optimal depth below the surfaceof the non-gaseous production being particlized to be dewatered.

In one embodiment, the present invention provides for an ultrasonicdewatering system for use in a gas producing well to particlizenon-gaseous production for removal from a wellbore, the ultrasonicdewatering system comprising:

an ultrasonic particle generator bank for positioning in the non-gaseousproduction for particlizing at least a portion of the non-gaseousproduction, the ultrasonic particle generator bank comprising:

an ultrasonic source for positioning at or below the surface of thenon-gaseous production for particlizing at least a portion of thenon-gaseous production; and

a power source in communication with the ultrasonic source.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the ultrasonic particle generator bank further comprisesa buoyancy control for positioning the ultrasonic source a desired depthbelow the surface of the non-gaseous production.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the desired depth is between about 0 mm and 4 mm.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the desired depth is about 1.0 mm or less.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the system or systems further comprise:

an electrical conveyance electrically connecting the power source andthe ultrasonic particle generator bank.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the electrical conveyance further comprises a lengthmanagement system for controlling the length of the electricalconveyance to maintain a length of the electrical conveyance at a lengthsuitable to maintain the buoyancy of the ultrasonic source at thedesired depth.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the ultrasonic particle generator bank comprisesmultiple ultrasonic sources and a transformer in communication with eachultrasonic source.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the wellbore further includes a velocity string mountedat a height that is raised above the surface of the non-gaseousproduction relative a typical velocity string.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the ultrasonic source is a directly coupled ultrasonicatomizer, a horn nebulizer, or a mesh nebulizer.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the wellbore is a horizontal wellbore.

In yet another embodiment, the present invention provides for anultrasonic dewatering system for use in a gas producing well comprisinga wellbore to particlize non-gaseous production for removal from thewellbore, the ultrasonic dewatering system comprising:

an ultrasonic particle generator bank in the non-gaseous production forparticlizing at least a portion of the non-gaseous production, theultrasonic particle generator bank comprising:

an ultrasonic source below the surface of the non-gaseous production forparticlizing at least a portion of the non-gaseous production; and

a power source in communication with the ultrasonic particle generatorbank.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the ultrasonic particle generator bank further comprisesa buoyancy control positioning the ultrasonic source a desired depthbelow the surface of the non-gaseous production.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the desired depth is between about 0 mm and 4 mm.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the desired depth is about 1.0 mm or less.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the system or systems further comprise:

an electrical conveyance electrically connecting the power source andthe ultrasonic particle generator bank.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the electrical conveyance further comprises a lengthmanagement system for controlling the length of the electricalconveyance to maintain a length of the electrical conveyance at a lengthsuitable to maintain the buoyancy of the ultrasonic source at thedesired depth.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the ultrasonic particle generator bank comprisesmultiple ultrasonic sources and a transformer in communication with eachultrasonic source.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the wellbore further includes a velocity string mountedat a height that is raised above the surface of the non-gaseousproduction relative a typical velocity string.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the ultrasonic source is a directly coupled ultrasonicatomizer, a horn nebulizer, or a mesh nebulizer.

In another embodiment of the ultrasonic dewatering system or systemsoutlined above, the wellbore is a horizontal wellbore.

In yet another embodiment, the present invention provides for a methodof dewatering a gas producing well comprising:

positioning a ultrasonic particle generator bank in the non-gaseousproduction of the wellbore; and

particlizing at least a portion of the non-gaseous production for upwardflow with the gas and eventual evacuation of the wellbore.

In yet another embodiment, the present invention provides for the use ofan ultrasonic device at a wellhead of a producing well, the ultrasonicdevice comprising

an ultrasonic source for positioning at the wellhead; and

a power source in communication with the ultrasonic source;

wherein the ultrasonic source is for particlizing water condensing atthe wellhead into ultrafine particles for re-vaporization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrative of one embodiment of anultrasonic dewatering system utilizing one embodiment of an ultrasonicdewatering device in a wellbore of a gas producing well; and

FIG. 2 is a schematic drawing illustrative of one embodiment of anultrasonic dewatering system utilizing one embodiment of an ultrasonicdewatering device in a wellbore of a gas producing well having avelocity string.

FIG. 3 is a plot showing surface tension energy of 1 L of pure water asa function of the droplet size;

FIG. 4 is a plot showing energy requirements of the commercial mistgenerators normalized to 1 L of water and superimposed with the scaledsurface tension energy curve;

FIG. 5 is a schematic illustration of the interfacial profile as acavitation bubble near the interface leads to its destabilization andhence pinch-off of an aerosol droplet.⁴;

FIG. 6 shows one embodiment of an ultrasonic atomizer from TDK;

FIG. 7 shows one embodiment of an ultrasonic fountain in water;

FIG. 8 is a schematics of one embodiment of an ultrasonic horn;

FIG. 9 shows an embodiment of an ultrasonic horn atomizer;

FIG. 10 shows an embodiment of a metal mesh nebulizer;

FIG. 11 shows liquid dispersing by an embodiment of a mesh nebulizer;

FIG. 12 A & B shows one embodiment of an experimental setup for testingthe mesh nebulizer wherein droplets are generated perpendicularly to themesh;

FIG. 13 is a chart illustrating evaporation rate from the mesh nebulizeras function of the depth with the mesh oriented at 45 degrees;

FIG. 14 shows one embodiment of a step horn, design #1;

FIG. 15 shows another embodiment of a step horn, design #2;

FIG. 16 shows yet another embodiment of a step horn, design #3;

FIG. 17 is a photograph showing atomization by step horn, design #4;

FIG. 18 is a plot showing a minimum gas (methane) velocity;

FIG. 19 is a plot showing the Reynolds number at minimum velocity;

FIG. 20 is a plot showing the gas flow profile in 4.5″ pipe whereV=V_(min), only droplets in the central zone are suspended (A);V_(min)=V_(bulk)=V_(max)/2, half of droplets can be lifted (B);V_(min)=V_(max)/10, 90% of droplets can be lifted (C)

FIG. 21 is a plot showing gas velocity capable of lifting 90% of waterdroplets

FIG. 22 is a schematic illustration of nebulizer lifted above the waterlevel to disperse water into CBM flow;

FIG. 23 is a schematic illustration of the placing of the meshnebulizers in a CBM well;

FIG. 24 is a schematic illustration of the lifting of the water to thelevel of the gas stream may optionally require spreading the water tothe pipe wall;

FIG. 25 is a schematic of one type of a horn type nebulizer in the 4.5″pipe; and

FIG. 26 is a schematic of a set of TDK type nebulizers in 4.5″ pipe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An ultrasonic device and system is provided for specific application tounloading non-gaseous production (typically mineralized water which mayor may not be associated with produced solids and/or hydrocarbonliquids) from gas producing wells. This process will be referred to as“dewatering” throughout this specification. Although the term“dewatering” is used, it will be appreciated that in addition to watergenerated by the hydrocarbon well, oil and/or condensates are alsoencompassed by this term and may be particlized and drawn upwards or inthe general direction of gas flow and evacuated from the gas producingwell.

It will be appreciated that reference herein to a “gas producing well”encompasses but is not limited to those wells that produce a gas,optionally in addition to a further hydrocarbon optionally in a liquidform or in the form of a liquid and solid composition such as an oilsand, wherein such gas generally has a sufficient pressure or density toflow from the formation to the surface.

It will also be appreciated that reference herein to an “ultrafine”particle encompasses those particles that are of a sufficient diameterto be transported by the displacement of the gas within the wellbore ofa gas producing well. Such particles include but are not limited tothose that behave in accordance to Stokes law. It will be appreciatedthat the diameter of the particles may vary based on the velocity and/ordensity of the gas in the wellbore.

One example of an ultrasonic dewatering system for use in a gasproducing well is illustrated in schematic form in FIG. 1. Theultrasonic dewatering system is shown installed in a wellbore 10 of agas producing well, for example a natural gas well. The wellbore 10 isshown as having a region wherein gas flows generally upwards from thebottom of the well and the gas producing reservoir and out of an outlet60 where it may be collected, filtered, separated, or otherwiseprocessed. The wellbore 10 includes a non-gaseous production 30generally collected towards the bottom or the lower region of thewellbore 10. The non-gaseous production 30 may be in the form of water,oil or condensates and generally a large portion of the non-gaseousproduction is in the form of water or mineralized water.

An ultrasonic particle generator bank 20 is positioned in thenon-gaseous production 30 for particlizing the non-gaseous production30. The non-gaseous particles are drawn upwards and out of the wellbore10 via the outlet 60 by the upward flow of the gas. This process may befacilitated by the production of smaller particles which are more easilydrawn upwards with the gas flow or which require a lower rate of gasflow to be drawn upwards for dewatering of the wellbore 10. Theultrasonic particle generator bank 20 shown in FIG. 1 is powered by apower source which may be positioned on the surface outside of thewellbore 10. It will be appreciated that a down bore power source, suchas a battery, may be used as a power source for the ultrasonic particlegenerator bank 20.

In the embodiment shown in FIG. 1, an electrical conveyance 50 transferspower from the power source 40 to the ultrasonic particle generator bank20. In one embodiment, the electrical conveyance 50 further includes alength management system for extending and withdrawing the conveyance asthe level of the non-gaseous production changes over time.

The ultrasonic particle generator bank 20 includes a transformer asneeded as the geometry of bank may vary depending on the down holeconfigurations. For example, the ultrasonic disk or disks may be alignedin a vertical orientation and strung vertically or at a desiredorientation. The ultrasonic particle generator bank 20 further includesan ultrasonic source, and generally multiple ultrasonic sources forproviding redundancy while increasing longevity and allowing forparticle formation rate control. The multiple ultrasonic sources may bepowered electrically from the surface, or by other means, for example abattery or battery pack. In addition, in a further embodiment, theultrasonic particle generator bank 20 includes a buoyancy control forcontrolling the depth at which the ultrasonic sources may be buoyedbelow the surface of the non-gaseous production. An optimal depth may beselected to more effectively particlize the non-gaseous production fordewatering. For example, the ultrasonic sources may be submerged about0-60 mm, or from 0-4 mm or from 1 mm or less below the surface of thenon-gaseous production depending on the type of atomizer, for example, adirectly coupled ultrasonic atomizer, a horn nebulizer, or a meshnebulizer. The buoyancy control is more effective when used togetherwith a length management system for the electrical conveyance so thatthe electrical conveyance does not significantly alter the buoyancy ofthe ultrasonic particle generator bank 20 by adding further weight or bypulling upwards on the ultrasonic particle generator bank 20.

Using the ultrasonic sources, ultrafine non-gaseous particles areproduced that can be transported with the gas phase out of the wellbore.It will be appreciated that the particlized non-gaseous particles havenot undergone a phase change and are not gas vapour but are non-gaseousparticles and so phase change of the particles from a gas vapour backinto a water droplet is not observed as the particlized non-gaseousparticles travel upwards with the gas flow. Without wishing to be boundby theory, it is predicted that some particles will coalesce on thewalls of the tubulars in the wellbore. However, in order forprecipitation to form in the free flowing space, it is predicted thatparticles will not spontaneously precipitate under static conditionsuntil they are about 500 microns in size, and to coalesce the dropletsshould be in a net downdraft condition and of varying droplet diameters.Since their targeted creational size is <30 microns the particles canimpact and join with a significant number of other particles before theyget to a raindrop size and precipitate in the wellbore. Since theparticles are in an updraft condition this will reduce their tendency toprecipitate. It will be noted that the higher the velocity the greaterthe momentum (proportional to velocity squared) and the greater thelikelihood that a collision will occur and will result in a coalescingof particles. There is therefore an increased risk of coalescingassociated with the use of velocity strings which is balanced off byreduced potential contact time associated with the increased velocity.The velocity should therefore be monitored to determine if flow volumehas decreased to a point where a velocity string is required. Atubingless open wellbore, for example, will typically have the lowestmomentum but the longest exposure time for potential collisions.Optimization will require the monitoring of the relationship betweenflow velocity and droplet growth.

It will be appreciated that dewatering of a gas producing well may becarried out both in gas producing wells of higher and lower gas flowvolume. In gas producing wells wherein gas flow velocity is lower,smaller particle size will allow for dewatering as lower gas flow willnecessitate a smaller particle size generation for allowing upward flowof the non-gaseous particles with the lower flow velocity of gas.

One typical method of dewatering includes the installation of a velocitystring in an effort to increase the rate at which gas flows upwards andout of the wellbore. The increase in gas flow velocity allows for thegas to carry the liquid or non-gaseous production upwards or in thegeneral direction of the flow of the gas such as either a traditionalvertical wellbore or a horizontal wellbore. As gas flow volumedecreases, a velocity string may be used to increase the velocity of thegas flow and continue operation of the gas producing well. An ultrasonicdewatering system may also be used in conjunction with a wellboreincluding a velocity string as shown in FIG. 2. A wellbore 10 includes avelocity string 70. As described with reference to FIG. 1, theultrasonic dewatering system includes an ultrasonic particle generatorbank 20 positioned in the non-gaseous production 30 and powered by apower source 40. The power source 40 may be connected to the ultrasonicparticle generator bank 20 by an electrical conveyance 50 if the powersource is external the wellbore 10 and may include a length managementsystem. In addition the ultrasonic particle generator bank 20 mayoptionally include a buoyancy control for positioning the ultrasonicsources at or near optimal depth in the non-gaseous production forparticle generation.

The particlized non-gaseous production flows upward with the gas flow upthe velocity string 70 where gas flow velocity is increased allowing fordewatering of the non-gaseous production in the wells of lower orreduced gas flow volume where a velocity string 70 has been installed.

Typically, a velocity string is positioned at the interface of thegas/fluid (top of perforations). One issue with the installation of avelocity string in a wellbore is that the velocity string can be floodedif the level of non-gaseous production rises higher than the bottom ofthe velocity string. By using an ultrasonic dewatering system thevelocity string may be raised so that the distance between the bottom ofthe velocity string 70 and the surface of the non-gaseous production isincreased or maximized. This reduces the risk of flooding of thevelocity string 70 but still allows for dewatering of the non-gaseousproduction in a particlized form.

It is predicted that an ultrasonic dewatering system may extend reservelife beyond those expected from reservoir energy unloading methods,install and operate more cheaply than pumps and provide continuousunloading as compared to periodic removal methods.

In addition to those applications outlined above, an ultrasonic particlegenerator for the particlization of the non-gaseous production forremoval of the non-gaseous production may also be applied to otherscenarios such as to blowdown to generate vapour (as compared toflashing by pressure drop). Ultrasonic particlization may also beapplied as a means for water treatment (similar to evaporationtechnology) wherein it may be used to create an ultrafine mist from theproduced water which would then require less energy to vaporize the mistthan the produced water itself.

Further, an ultrasonic particle generator may be used in combinationwith steam generators, which may be distant, for example severalkilometers, from the wellhead. An ultrasonic particle generating may bepositioned at or near the well head to convert condensed water(generally at very high pressure), which forms when the steam travelslong distances in pipes and cools. The ultrasonic device could convertthe condensed water to ultrafine particles. The energy within the steamflowing in the pipe may be sufficient to re-vaporize the ultrafineparticles at the well head back to steam, thus increasing the steamquality being injected into the wellbore. Ultrasonic devices may beplaced along the pipe at locations where condensation is typicallyobserved.

Furthermore, ultrasonic particlization of the non-gaseous production maybe used at tailings ponds, in an oil-water separation applicationwherein ultrasonic devices may be used on or near the surface to causecoalescence of emulsions and enhance separation. The ultrasonic particlegenerator may be operated to generate particles of water leaving the oilin a liquid state, or vice-versa.

In addition, an ultrasonic particle generator may be used to generate amist within a steam separator to increase the surface-area of droplets,and thus induce more evaporation of vapour from the blowdown liquid.

Further, an ultrasonic particle generator may be employed by a lowerpressure mist generator vessel after the steam separator to recover alow-pressure water vapour stream from the hot boiler blow-down. Such anapplication may be used to bring the blowdown flashing step closer tothermodynamic equilibrium by increasing the liquid surface area.

In addition, an ultrasonic particle generator may be used whenintroducing diluent into process emulsion (before a free-water knockoutand/or treater) to finely disperse condensate as a “pseudo-mist in aliquid”. This may reduce localized areas of high condensateconcentration that could lead to asphaltene precipitation from thebitumen.

It will be appreciated that various modifications, revisions and/oradditions may be made to the systems and methods outlined herein withoutdeparting from scope of the invention and these modifications, revisionsand/or additions are within the contemplated scope of the invention. Inaddition, it will be appreciated that the embodiments outlined above aremerely illustrative of various contemplated embodiments and are notintended to be limiting in any way.

Experimental Work

It will be appreciated that the following experimental section isdirected to illustrative embodiments of various aspects of the inventionwhich are not intended to be limiting but merely illustrative of theconcept of particlization or atomization of a liquid such as water usingvarious ultrasonic devices and implementation methods. The invention isnot limited by these devices and methods or the implementationtechniques outlined in this section. All indication of volume,performance, orientation, dimensions, power output and materials issimply intended as illustrative and should not be deemed as essential oras a promise or guarantee or utility or performance.

Mist Generation Starting Point: Calculated Energy for Water Dispersion

The energy required for mechanical generation of the mist from water canbe calculated based on surface tension. The surface tension of the bulkwater which is contained in a volume unit (1 L) is fairly low due tosmall surface area. The surface area is increased while this volume isdispersed into miniature droplets. The volume of the ordinary droplet isproportional to R³, subsequently the number of droplets is changed as1/R³, where R is the radius of the droplet. As the surface of theordinary droplet is proportional to R², the total surface area of themist is changed with R as 1/R. Thus the finer the mist, the more energyis required to produce it. FIG. 3 represents the absolute surfacetension energy of the 1 L of water as a function of the droplet size.For practicality, the data is presented in kW*h as oppose to scientificvalues in Joules.

Real Power Consumed, Real Efficiency

The calculated data in FIG. 3 is fairly optimistic; only 0.04 W*h isneeded for dispersing of 1 L of water into a 3 μm mist. This representsonly 0.8 W of power for permanent evaporation of required 20 L per hourwhich is the power of one LED lamp. The practical data is very differentthough.

The Table 1 presents data for several commercial ultrasonic mistgenerators.

Mist Power, Mean droplet rate, Manufacturer W diameter, μm L/h Note TDK30 3 0.45 1.6 MHz TDK 13 2 0.20 2.4 MHz STULZ-Ultrasonic 495 3 10 NewTech Trading 1800 3 18 Hangzhou Success 500 62 150 YPW63, 15 kHzUltrasonic 300 51 70 YPW61, 20 kHz Equipment Co. 100 39 50 YPW59, 30 kHz60 32 10 YPW57, 40 kHz 30 28 0.5 YPW51, 50 kHz 30 28 2 YPW55, 50 kHz

FIG. 4 superimposes power of the listed above devices normalized to oneliter of water and scaled to the surface tension energy curve. The scalefactor is 1700 giving us efficiency of about 0.06%.

Efficiency

The estimated efficiency of less than 0.1% is in accordance with datapresented in the recent monograph.²

The heat vaporization of pure water is 2260 kJ/kg which is equivalent to627 W*h or 0.6 kW*h. That is still much higher than any real mechanicalatomizer consumes. Another words, ultrasonic atomizers are much moreefficient than thermal vaporization.

The mechanism of ultrasonic atomization has not been fully understoodyet. This phenomenon is fairly complex from an engineering point of viewas it is influenced by properties of liquid, its amount and temperature,geometry of the transducers, operating frequency, applied voltage, etc.The complexity is coming from a not-fully known physics of theatomization process. According to the classic hypothesis, the generationof the aerosol droplets involves cavitation which occurs when vaporbubbles are formed in the liquid as the pressure in a localized regionof the fluid suddenly decreases owing to the periodic disturbancesintroduced by the sound excitation.² If the local pressure falls belowthe vapor pressure during the negative half cycle of the oscillation,the liquid in that region essentially “boils” to form a vapor pocket. Onthe positive half cycle of the oscillation, these bubbles suddenlycollapse with such intensities that extremely high instantaneouspressures and accelerations are generated in the form of shock waves.According to a second theory, the disturbance shock waves resulting fromthe implosion of the bubbles during cavitation lead to the excitation offinite amplitude capillary waves that result in droplet ejection.³ Thistheory dominates in explanation of the aerosol formation by ultrasonictransducers placed in the liquid and is illustrated in FIG. 5.

Whichever mechanism is responsible for the excitation of capillary waveon the interface, a critical threshold amplitude of the waves must beexceeded before the onset of nebulization resulting from theirdestabilization. The critical amplitude α_(c) is given by:

$\alpha_{c} = {\frac{2\mu}{\rho}\left( \frac{\rho}{\pi \; \gamma \; \omega} \right)^{\frac{1}{3}}}$

Where μ, ρ, γ—is the viscosity, density and surface tension of theliquid and ω=2πf. For water at 10° C. to 20° C. and frequency f of 10kHz, 100 kHz, and 1 MHz, the critical amplitude α_(c) will be equal to0.13, 0.06 and 0.03 μm, respectively.

Ultrasonic Versus Hydraulic Atomization

A hybrid solution which represents ultrasonic atomizer together with theair supply (or, in general, gas carrier) is effectively used inultrasonic sprayers providing satisfactory droplets size and largevolume rate. However, this solution may not be practical for UDWW due tosubstantial gas consumption.

Another option is hydraulic dispersion in which the liquid is passedunder considerable pressure through a nozzle. The pressure in hydraulicatomizers is about 80 to 100 bar. The advantage of the hydraulicdispersion is its high efficiency. In fact, this is the most efficientmethod of water atomization as it allows dispersing of 1 m³ of water byusing only 2 kW of power.5

The practical merit of such an economical approach, however, requiresfurther investigation due to the need of having a miniaturehigh-pressure downhole hydraulic pump. Also, the droplet size may varyand as result Prof. R. Christiansen observed about 70% of hydraulicallyformed water droplets were stuck to the pipe wall.¹

Shalunov A. V. claimed achieving the efficiency above 0.5% by directingthe water stream horizontally on a vertically placed large (42 cm) piezodisk.⁵ However, this approach is not applicable for ultrasonic wellboredewatering devices (UWDD).

Methods for Ultrasonic Mist Generation

Ultrasonic atomizers can be divided in three groups:

-   -   Transducers immersed in the liquid, direct energy transfer;    -   Transducers displaced from the liquid, the energy is transferred        trough special attachments (horns); and    -   New type of nebulizers with mesh actuators.

Transducer Immersed in the Liquid

An ultrasonic atomizer developed by TDK was used. Direct coupling wasused between the piezo element and the vaporizing liquid (water).

The method requires an active control of water level above theultrasonic transducer to avoid overheating the piezo element. Theoptimum level is from 40 to 45 mm. A very similar product is offered bythe APC with optimum water level of about 30 mm. The atomization rate isfrom 300 to 500 mL/h, as illustrated in FIG. 6.

This type of ultrasonic nebulizers has been popular in medicalapplication for several decades. The ultrasonic fountain in the water(FIG. 7) can be cost-efficiently achieved, and it is widely used fordecoration.

The method, however, has not found any valuable application in theindustry. The reason is its limited power. The transducer cannot besimply scaled up to get higher atomization rate as water startssporadically dispersing thus reducing the water level from its optimum.The atomizer becomes a demonstrator of a volcano eruption.

Placing number piezo elements of 20 mm to 25 mm in diameter allows aparallel work of several transducers simultaneously. That is onepotential approach for UWDD. The number of elements is limited though bythe pipe diameter, about 22 of elements will fit into the 4.5″ pipe.

Transducers Displaced from the Liquid, Half Wave Couplers

High volume atomizers can be built based on coupling the piezo elementto the liquid via a probe (a horn) which length is equal to half of awavelength. The probe is made from low-density metals such aluminum ortitanium. These types of atomizers operate at a frequency range from 20to 100 kHz, consequently the probe length is from 2.5 to 12.5 cm. Hornsare designed to achieved the maximum vibration amplitude at the tip(FIG. 8).

Most of technological ultrasonic equipment such as welders, sprayers,mixers, etc. is built this way. The horn is bolted to a powerfulLangevin ultrasonic transducer. For atomizing, the liquid is deliveredthrough the transducer or through the base of the horn as shown in FIG.9.

Some horn atomizers have the liquid feed channel passing near the nodeinstead of going along the axis of the device.

Horns transfer the power of transducers into a smaller area at the horntip. As oppose to direct coupling, all processes occur in the proximityof the horn surface. The active depth depends on amplitude of thevibration and is typically less than 1.0 mm.

Thin Plate Couplers, Mesh Nebulizers

Piezo elements operate more efficiently in a radial mode rather than inaxial mode. Conversion of the radial vibration into axial displacementis therefore necessary. The most compact solution can be achieved usinga metal mesh nebulizer. The active element in this device represents athin (0.1-0.2 mm) metal foil with a number of miniature holes (10 to 200μm diameter). The foil is placed between two piezo elements as shown inFIG. 10.

Radial vibration of thin piezo elements translates into the longitudinaldisplacement of the foil. Small amount of water placed on the top of thefoil is dispersed effectively by the vibration; the mesh structure ofthe foil enhances the effect due to the surface acoustic waves (SAW), asshown in FIG. 11. Such design is used in miniature, low-power consumersprayers.⁶

The size of actuators may be about 12×12 mm, the mesh nebulizers cannotbe scaled up as they disperse water only at the edge of the mesh. Fullcovering of the mesh even with a thin water layer will result inimmediate interruption of the atomization due to disappearance of theSAW.

Testing Metal Mesh Nebulizer

A metal mesh nebulizer with a 12×12 mm mesh was fixed in a 3D opticaltranslation stage as shown in FIG. 12. The nebulizer was powered from acustom generator with tunable frequency from 100 to 150 kHz. Thepeak-to-peak voltage was constant and equal to 12V. A water containerwas placed on a digital scale with resolution of 0.01 g. The nebulizerwas oriented at 45 degree angle to the water surface.

FIG. 13 shows the experimental data on evaporation rate as a function ofthe depth. The maximum rate was recorded at the depth of 0.3 mm whichwas equal to 1.05 g/min. Deeper placement of the mesh into the waterdrastically reduced the atomization. The evaporation was entirelystopped at the depth of less than 1.0 mm. The maximum consumed power was0.75 W and it remained practically unchanged with the depth. Generateddroplets were oriented perpendicularly to the mesh.

Changing the orientation of the mesh from 45 degrees reduced theevaporation rate. The maximum rate dropped to 0.5 g/min at 30 degree andbelow 0.2 g/min at 60 degree. Placing the mesh horizontally stopped theevaporation entirely.

The data above is equivalent to the maximum Watt*Hour production of 84g. Evaporation of 1 L and 20 L of water will need 12.0 and 240 W*h,respectively. Therefore, the mesh nebulizer appears to be about 5 timesmore efficient than the direct coupled nebulizer from TDK.

Horn Atomizers

The first model of the step-type horn was prototyped from 6061 aluminum,the diameter of the bottom and upper parts was 25.4 and 12.7 mm,respectively. The horn was firmly bolted to a 40-kHz 60 W Langevintransducer. The transducer was powered with 300 Vp-p. An acrylic pipewas mounted in the location of the node (FIG. 14). The pipe was filledwith water to simulate water collection from the pipe wall.

The vaporization started after reducing the water layer to approximately1.0 mm above the tip. The atomization was accompanied with sporadiceruptions which were spreading large water aggregates. The appearance ofthe tip was changed from shiny & polished to dull due to degradation ofthe surface under cavitation.

A second horn was machined with an extension at the tip, FIG. 15. Theextension had the same diameter as the bottom part of the horn, 25.4 mm.

The second horn had a secondary resonance peak at approximately 45 t kHzwhich was the result of superposition of the original 40 kHz transducerpeak and overtones of the conical extension of the horn.

The water was delivered on the tip from the top. The second horndispersed the water more intensively; however, the intensity depended onlocation of the water and its amount. Larger droplets of water werecollected closer to the center without getting dispersed for a longtime.

The third step horn has a delivery channel through the center as shownin FIG. 16. The radial part of the channel was drilled close to the stepwhere minimal displacements are taking place. The outer dimension of thehorn matched the transducer diameter of 44.40 mm (1.75″) and the thinnerpart was 22.20 mm. The horn ended up with a 150-degree cone.

Preliminary testing shows that the main transducer resonance of 40 kHzsplit by several overtones. The water dispersing depended on the amountof water and orientation of the setup. A slight deviation of the hornfrom the vertical orientation changed the amount of water on thecorresponding side of the cone and that was accompanied with thesporadic atomization. The test indicated, therefore, that taking morepower from the transducer does not necessarily mean a better performanceof the nebulizer if the power is spread over the large dispersing tip.

The prototyping of the forth horn included the modification of the basic28-kHz Langevin transducer. Its base was machined out to OD of 25 mm atthe length of 26 mm. The step horn had same diameter at the bottom (20mm long) and 12.5 mm diameter upper part. The delivery channel of 2.0 mmdiameter was drilled along the axis, its radial part was machined closeto the location of the step, FIG. 17.

Atomization from the forth step horn was the most consistent from allhorn type nebulizers above. The resonance frequency increased to 33 kHz,such a high shift was not anticipated based on modeling in Solid Worksalthough it may be influenced by reducing the counter-mass of thetransducer and location of the delivery channel. The measured actualpower was 24 W and the dispersion rate was near 10 mL/min or 600mL/hour. This efficiency was better than one can get from the TDK typeof nebulizers although it was still lower than we achieved from the meshnebulizers.

The horn type nebulizers are particularly sensitive to horn geometry,type of transducers and their coupling to the horn, material, etc. Theycan reach efficiency of 0.1% by generating water droplets of 30 μm.5 Theevaporation rate of 20 L/hour will require only 40 W of power which isless than 1/30 of the standard TDK nebulizers. Scaling up the power isnot exactly linear, one should expect the real output of the horn typenebulizers as 150 W for 20 L/hour.

Gas Lift Droplet Size and Minimum Gas Flow

Condensed water can be lifted by methane up to the wellhead according tothe UWDD concept. This paragraph is devoted to the estimation of theminimum gas velocity needed for lifting the water droplets. Suchvelocity can be directly calculated as a terminal velocity from Stokeslaw:

$V_{m\; i\; n} = {\frac{2}{9}\frac{\rho_{w} - \rho_{g}}{\mu}g\; r^{2}}$

Where: ρ_(w) is the density of the water; ρ_(g) is the density of thegas; μ is the dynamic viscosity of the gas; g is the gravitationalacceleration; r is the radius of the droplet.

Properties of pure methane under normal conditions (temperature 20° C.and atmospheric pressure) are: ρ_(g)=0.664 kg/m³; μ=1.1*10⁻⁵ Pa s.

Calculated minimum gas velocity is presented in FIG. 18 as a function ofthe droplet size.

Droplets will be suspended in the gas which moves with minimum velocityV_(min). The flow regime is expected to be laminar due to low value ofV_(min) and a small pipe diameter. Reynolds number for a bulk velocityof V_(min)/2 and low amount of water droplets in the flow is equal to:

${Re} = {\frac{V_{m\; i\; n}}{2\mu}D\; \rho_{g}}$

and is plotted in FIG. 19.

As it seen from FIG. 19, flow regime will be laminar over the entirerange of droplet size. This means that a correction factor×2 should beapplied for V_(min) to account for the bulk velocity. Otherwise, only acentral part of the gas flow will be able to suspend droplets (FIG. 20).

Assuming the uniform distribution of droplets in the pipe, the gas flowof V=2Vmin (correction factor×2) should lift half of droplets in thepipe. Correspondently, selecting the correction factor×10 should assurelifting of 90% of all droplets.

FIG. 21 presents calculated data of Vmin multiplied by the correctionfactor×10. The underlined value of 2.0 m/s corresponds to droplet sizeof approximately 60 μm.

The required dewaterization rate of 20 L/h increases the density of themixture by approximately 0.27 kg/m³ at velocity of 2.0 m/s. Thiscorresponds to a higher Reynods number of about 18,000 which indicatesthat flow regime become turbulent. Turbulence increases the frictionbetween gas and water droplet by a factor C_(t):⁷

C _(t)=1+0.16Re ^(3/2)

Correspondently, one should expect a better gas lifting due toturbulence when the water content increases.

Location of the Nebulizer

Gas flow in the CBM well begins from the lowest level of the coal seamwhich is located above the bottom of the gas producing well. As thedispersed water droplets have almost the same temperature as thesurrounding gas, they do not spread too far from the nebulizer withoutbeing picked up by the gas flow. Therefore, the nebulizer should belifted up to the level where gas flows at least with minimum velocityV_(min), FIG. 22.

Lifting the nebulizer to the height H from the water surface (or abottom of the gas producing well) may be done using for example a smallwater pump producing at least 20 L/h at pressure of a few bars.

Practical Considerations, Recommendations and Conclusions MeshNebulizers

Mesh nebulizers represent an elegant and efficient ultrasonic atomizingsolution. They operate at frequencies above 100 kHz, which allowgeneration of quite small and easy to lift water droplets.

Several rows of nebulizers may be required to achieve a desiredparticlization rate by placing the nebulizers along the pipe wall (FIG.23).

Mesh nebulizers generate water mist generally perpendicular to the mesh,and a 45 degree orientation of the mesh was found to be optimal in termsof generating efficiency. Therefore, the mist will be directed under 45degree to the pipe instead of being dispersed vertically up. One shouldexpect some mist deposition on the pipe wall before some droplets willbe drafted up by the gas flow.

3) Water lifted from the bottom at the height H may be dispersed to thepipe wall in order be picked up and dispersed by the mesh actuators(FIG. 24)

Horn Nebulizers

Horn nebulizers are a robust and versatile atomizing tool. They candisperse high power within a limited volume and they do not suffer fromoverheating when the water level is low. Horn type nebulizers generallypair with a pump, such as a micropump, as they do not disperse bulkwater. However, the pump is needed in the UDWW anyway for lifting thewater level at the height H. In one embodiment, the nebulizer may bepositioned in the center of the pipe. It will occupy the space ofapproximately 70 mm in diameter which is approximately 40% of the pipecross section (FIG. 25.)

High-Frequency Direct Coupling

High-frequency (for example 1.6 or 2.4 MHz) ultrasonic atomizers basedon direct coupling of piezo elements to the water provides the tiniestwater droplets which can be lifted by very low gas flow as shown in FIG.26. Their maximum power is generally limited. Given a production rate ofa single unit of approximately 0.5 L/hour, a total of 40 devices wouldbe needed for a volume of 3 bbd.

REFERENCES

-   1. Christiansen R. L. New Technologies for Lifting Liquids from    Natural Gas Wells, report for DOE, Colorado School of Mines, 2003-   2. Plesset M. S., Prosperetti A. “Bubble Dynamics and Cavitation”.    Annu. Rev. Fluid Mech. 1977, 9; 145-185-   3. Boguslayskii Y. Y., Ekhadyosants O. K. “Physical Mechanism of the    Acoustical Atomization of a Liquid” Sov. Phys. Acoust. 1969, 15,    14-21.-   4. Boulton-Stone J. M., Blake J. R. “Gas Bubble Bursting at a Free    Surface” J. Fluid Mech. 1993, 254: 457-66.-   5. Khmelev V. N., Shalunov A. V., Shalunova A. V. “Ultrasonic    Dispersion of Liquids”—Altai Tech Univ., 2010, (in Russian).-   6. Yeo L. Y., Friend J. R., etc. “Ultrasonic Nebulization Platforms    for Pulmonary Drug Delivery”. Expert Opin. Drug Deliv., 2010, 7:    663-679.-   7. Van Boxel J. H. “Numerical Model for the Fall Speed of Raindrops    in a rainfall simulator”—Workshop on Wind and Water Erosion,    1997, p. 77-85

What is claimed is:
 1. An ultrasonic dewatering system for use in a gasproducing well to particlize non-gaseous production for removal from awellbore, the ultrasonic dewatering system comprising: an ultrasonicparticle generator bank for positioning in the non-gaseous productionfor particlizing at least a portion of the non-gaseous production, theultrasonic particle generator bank comprising: an ultrasonic source forpositioning at or below the surface of the non-gaseous production forparticlizing at least a portion of the non-gaseous production; and apower source in communication with the ultrasonic source.
 2. Theultrasonic dewatering system according to claim 1, wherein theultrasonic particle generator bank further comprises a buoyancy controlfor positioning the ultrasonic source a desired depth below the surfaceof the non-gaseous production.
 3. The ultrasonic dewatering systemaccording to claim 2, wherein the desired depth is between about 0 mmand 4 mm.
 4. The ultrasonic dewatering system according to claim 2,wherein the desired depth is about 1.0 mm or less.
 5. The ultrasonicdewatering system according to claim 1, further comprising: anelectrical conveyance electrically connecting the power source and theultrasonic particle generator bank.
 6. The ultrasonic dewatering systemaccording to claim 5, wherein the electrical conveyance furthercomprises a length management system for controlling the length of theelectrical conveyance to maintain a length of the electrical conveyanceat a length suitable to maintain the buoyancy of the ultrasonic sourceat the desired depth.
 7. The ultrasonic dewatering system according toclaim 1, wherein the ultrasonic particle generator bank comprisesmultiple ultrasonic sources and a transformer in communication with eachultrasonic source.
 8. The ultrasonic dewatering system according toclaim 1, wherein the wellbore further includes a velocity string mountedat a height that is raised above the surface of the non-gaseousproduction relative a typical velocity string.
 9. The ultrasonicdewatering system according to claim 1, wherein the ultrasonic source isa directly coupled ultrasonic atomizer, a horn nebulizer, or a meshnebulizer.
 10. The ultrasonic dewatering system according to claim 1,wherein the wellbore is a horizontal wellbore.
 11. An ultrasonicdewatering system for use in a gas producing well comprising a wellboreto particlize non-gaseous production for removal from the wellbore, theultrasonic dewatering system comprising: an ultrasonic particlegenerator bank in the non-gaseous production for particlizing at least aportion of the non-gaseous production, the ultrasonic particle generatorbank comprising: an ultrasonic source below the surface of thenon-gaseous production for particlizing at least a portion of thenon-gaseous production; and a power source in communication with theultrasonic particle generator bank.
 12. The ultrasonic dewatering systemaccording to claim 11, wherein the ultrasonic particle generator bankfurther comprises a buoyancy control positioning the ultrasonic source adesired depth below the surface of the non-gaseous production.
 13. Theultrasonic dewatering system according to claim 12, wherein the desireddepth is between about 0 mm and 4 mm.
 14. The ultrasonic dewateringsystem according to claim 13, wherein the desired depth is about 1.0 mmor less.
 15. The ultrasonic dewatering system according to claim 11,further comprising: an electrical conveyance electrically connecting thepower source and the ultrasonic particle generator bank.
 16. Theultrasonic dewatering system according to claim 15, wherein theelectrical conveyance further comprises a length management system forcontrolling the length of the electrical conveyance to maintain a lengthof the electrical conveyance at a length suitable to maintain thebuoyancy of the ultrasonic source at the desired depth.
 17. Theultrasonic dewatering system according to claim 11, wherein theultrasonic particle generator bank comprises multiple ultrasonic sourcesand a transformer in communication with each ultrasonic source.
 18. Theultrasonic dewatering system according to claim 11, wherein the wellborefurther includes a velocity string mounted at a height that is raisedabove the surface of the non-gaseous production relative a typicalvelocity string.
 19. The ultrasonic dewatering system according to claim11, wherein the ultrasonic source is a directly coupled ultrasonicatomizer, a horn nebulizer, or a mesh nebulizer.
 20. The ultrasonicdewatering system according to claim 11, wherein the wellbore is ahorizontal wellbore.
 21. A method of dewatering a gas producing wellcomprising: positioning a ultrasonic particle generator bank in thenon-gaseous production of the wellbore; and particlizing at least aportion of the non-gaseous production for upward flow with the gas andeventual evacuation of the wellbore.
 22. Use of an ultrasonic device ata wellhead of a producing well, the ultrasonic device comprising anultrasonic source for positioning at the wellhead; and a power source incommunication with the ultrasonic source; wherein the ultrasonic sourceis for particlizing water condensing at the wellhead into ultrafineparticles for re-vaporization.